1. Field of the Invention
This invention relates to a useful metal-encapsulated fullerene compound having application as a functional material, a superconducting material, an electronics material or a pharmaceutical material.
2. Description of the Related Art
An all-carbon fullerene is a recently discovered carbon allotrope typified by Buckminsterfullerene (C.sub.60), and it has attracted attention in recent years because of its unique cage (shell) structure. After discovery of macroscopic preparations of all-carbon fullerenes in 1990, various physical and chemical properties of the fullerenes were clarified, and since many derivatives of all-carbon fullerene have now been synthesized, nowadays they are applied to electrical conductors, semiconductors and pharmaceutical products. Many reports about application of all-carbon fullerenes and their derivatives have appeared in the literature (Gendaikagaku, No. 253, April 1992, pp. 12-19, and Kagaku, Vol. 50, June 1995, pp. 12-16), and in particular, biological activity have emerged for derivatives of all-carbon fullerene (J. Am. Chem. Soc., 1993, Vol. 115, p. 6506) which have led to further research.
A metal-encapsulated fullerene is a new material closely related to an all-carbon fullerene. This material is comprised of an ordinary fullerene cage and a metal atom incorporated into inside of its shell structure. The physical and chemical properties of metal-encapsulated fullerenes can be controlled by changing their encapsulated elements, suggesting many potential applications. After the invention of a method for macroscopic synthesis of lanthanum-encapsulated fullerenes La.sub.m @C.sub.n (R. Smalley et al., J. Phys. Chem. 1991, Vol. 95, p.7564), intensive research is now being carried out. According to Smalley's report, it is possible to produce one-lanthanum-encapsulated fullerenes La@C.sub.n with n=36-122, two-lanthanum-encapsulated fullerenes La.sub.2 @C.sub.n up to n=110, and three-lanthanum-encapsulated fullerenes La.sub.3 @C.sub.n up to n=98. Although there is essentially no higher limit, there is a lower limit for the value of n in M.sub.m @C.sub.n fullerenes. And it is thought that the limit depends on the size and number of encapsulated metals or metal ions.
In the case of La@C.sub.82, a method has been described for obtaining a pure sample and its characterizations have been reported (K. Kikuchi et al., Chem. Phys. Lett. 1993, Vol. 216, p. 67 and J. Am. Chem. Soc. 1994, Vol. 116, p. 9367; T. Suzuki et al., J. Am. Chem. Soc. 1993, Vol. 115, p. 11006; K. Yamamoto et al., J. Phys. Chem. 1994, Vol. 98, p. 2008 and J. Phys. Chem. 1994, Vol. 98, p. 12831). In the case of fullerenes encapsulating two lanthanum atoms, the characterization of La.sub.2 @C.sub.80 has been reported (T. Suzuki et al., Angew. Chem. Int. Ed. Engl. 1995, Vol. 34, p. 1094).
The synthesis of scandium-encapsulated fullerenes, Sc.sub.m @C.sub.n (m=1-3), is reported by H. Shinohara in Nature 1992, Vol. 357, p. 52. The separation methods are described for Sc.sub.2 @C.sub.n (n=74, 82, 84) in J. Phys. Chem. 1993, Vol. 97, p. 4259, and for Sc.sub.3 @C.sub.82 in J. Phys. Chem. 1994, Vol. 98, p. 8597.
The first synthesis of yttrium-encapsulated fullerenes, Y.sub.m @C.sub.n, was reported by J. Weaver et al. (Chem. Phys. Lett. 1992, Vol. 190, p. 460), and the characterization of Y@C.sub.82 was described by K. Kikuchi et al. in J. Am. Chem. Soc. 1994, Vol. 116, p. 9367.
The production and extraction of lanthanoid-encapsulated fullerenes, M.sub.m @C.sub.n (M=Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) are described by R. L. Whetten et al. in J. Phys. Chem. 1992, Vol. 96, p. 6869, and the production of other lanthanoid-encapsulated fullerenes, M.sub.m @C.sub.n (M=Pr, Eu, Yb, Lu) is reported by R. S. Ruoff et al. in J. Phys. Chem. 1993, Vol. 97, p. 6801. Of these, the isolation and characterization of Gd@C.sub.82 were reported by H. Funasaka et al. in Chem. Phys. Lett. 1995, Vol. 232, p. 273 and J. Phys. Chem. 1995, Vol. 99, p. 1826.
As to an actinoid-encapsulated fullerene, the synthesis of uranium-encapsulated fullerenes U.sub.m @C.sub.n (m=1-2, n=28-80) is reported by R. Smalley et al. in Science 1992, Vol. 257, p. 1661.
In the case of alkali-metal-encapsulated fullerenes, potassium and cesium have been reported. The production of potassium-encapsulated fullerenes, K@C.sub.n, is reported by R. Smalley et al. in J. Am. Chem. Soc. 1988, Vol. 119, p. 4464, and of cesium-encapsulated fullerenes, Cs@C.sub.n, by R. Smalley et al. (J. Am. Chem. Soc. 1988, Vol. 110, p. 4464).
Fullerenes encapsulating an alkaline earth metal have been reported for calcium, strontium and barium. The production and extraction of a calcium-encapsulated fullerene, Ca@C.sub.60, was first performed by R. Smalley et al. (Chem. Phys. Lett. 1993, Vol. 207, p. 354 and Z. Phys. 1993, Vol. D26, p. 297), after which the production of other Ca@C.sub.n fullerenes was confirmed by K. J. Fisher et al. (J. Chem. Soc., Chem. Commun. 1993, 941). The production of strontium-encapsulated fullerenes, Sr.sub.m @C.sub.n, and barium-encapsulated fullerenes, Ba.sub.m @C.sub.n, was reported by K. J. Fisher et al. (J. Chem. Soc., Chem. Commun. 1993, 1361).
As an example of fullerenes encapsulating a transition metal, the synthesis of an iron-encapsulated fullerene, Fe@C.sub.60, is reported by C. N. R. Rao et al. (J. Am. Chem. Soc. 1992, Vol. 114, p. 2272, and Indian J. Chem. 1992, Vol. 31, p. F17). The first synthesis of a cobalt-encapsulated fullerene, Co@C.sub.60, is described by D. S. Bethune et al. in Nature 1993, Vol. 363, p. 605.
As described hereinabove, it has been reported that a variety of metal elements are encapsulated in fullerenes, most of the reports concern methods of their production, isolation and characterization. Since all-carbon fullerene derivatives have biological activity, various potential applications of metal-encapsulated fullerenes, such as biological activity, are now envisaged. In addition, in the case of missile therapy of cancer using radioactive elements, fullerenes encapsulating radioactive elements are expected to play an important role. When you test biological activity of a certain substance or use it in a treatment for cancer, its water solubility must be enhanced and its affinity for the organism must be increased. Therefore it is necessary to synthesize derivatives of metal-encapsulated fullerenes with hydrophilic groups in the same way as the case for all-carbon fullerenes. To date, however, no metal-encapsulated fullerene derivative has been successfully synthesized in test tube amounts, and its method of synthesis remains unclear. This is why there have so far been no reports for the biological activity of metal-encapsulated fullerene compounds, and why there have so far been no successful development and no synthesis of functional materials, semiconductor materials, electronics materials and pharmaceutical materials using these metal-encapsulated fullerenes or metal-encapsulated fullerene compounds at the present time.